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Do I need to create the datafile first before I can create the TABLESPACE? Are there pre-existing datafiles in the database that I can use? What would be the proper syntax to create a tablespace and/or a datafile? How and where do I create a .dbf file to create the tablespace?

It sound like you are new to Oracle databases. Oracle provides a wealth of documentation at http://tahiti.oracle.com/. Specifically, I recommend reading their excellent introduction to the database called the Concepts Guide.